Conventionally in an alkali storage battery such as a nickel-zinc storage battery, a nickel-cadmium storage battery, or a nickel-metal hydride storage battery, nickel hydroxide has been generally used as a cathode active material for its positive electrode.
In the case of the alkali storage battery thus using nickel hydroxide as a cathode active material for its positive electrode, electrochemical reversibility is very superior. However, oxygen gas is easily generated from the positive electrode during the charging. Consequently, this oxygen gas evolution makes it difficult for batteries to charge.
In addition, the pressure rises in the battery by oxygen gas evolution, so that an electrolyte solution (hereinafter referred to as an electrolyte) leaks from the battery, and charge/discharge cycle performance is degraded.
In the above-mentioned alkali storage battery, various methods have been conventionally considered. For example, a void inside the battery has been enlarged in order to prevent the pressure in the battery from rising by oxygen gas evolution, and the area of its negative electrode has been increased in order to absorb oxygen gas at the negative electrode.
In such a case, however, there are some problems. For example, the energy density of the alkali storage battery is decreased. Therefore, a battery capacity cannot be obtained sufficiently.
On the other hand, when manganese dioxide electrode is used for a positive electrode of an alkali manganese battery, it is possible to suppress the oxygen gas evolution during the charging. However, the reversibility of manganese dioxide electrode is very poor.
There have been conventionally various developments in order to improve characteristics in an alkali storage battery using nickel hydroxide as an active material for its positive electrode as described above.
Proposed as the developments include one where at least one type of manganese, silver, cobalt and their compounds are mixed with nickel hydroxide to produce a large-capacity nickel electrode, as disclosed in JP, 54-4334, A; one containing a manganese compound in a positive electrode to obtain a battery which is hardly self-discharged and has a low life, as disclosed in JP, 5-121073, A; one containing at least one type of cadmium, calcium, zinc, magnesium, iron, cobalt and manganese in an active material composed of powdered nickel hydroxide, to inhibit swelling of a positive electrode and improve the energy density and the cycle life, as disclosed in JP, 5-21064, A; and one where that not less than 50% of manganese exists in a state where its valence is 3 in nickel hydroxide containing manganese, to improve the active material utilization of nickel hydroxide, as disclosed in JP, 7-335214, A.
Even in the developments disclosed in the above-mentioned publications, a method of effectively suppressing oxygen gas evolution during the charging is not disclosed. Therefore, the pressure in the battery still rises by oxygen gas evolution, so that there are some problems. For example, an electrolyte leaks from the battery, and charge/discharge cycle performance is degraded.
An object of the present invention is to solve the above-mentioned problems in an alkali storage battery such as a nickel-zinc storage battery, a nickel-cadmium storage battery, or a nickel-metal hydride storage battery using nickel hydroxide as a cathode active material for its positive electrode.
Specifically, an object of the present invention is to make it possible to simply suppress, in an alkali storage battery using nickel hydroxide as a cathode active material for its positive electrode, oxygen gas evolution during the charging of the battery.
Another object is to obtain an alkali storage battery having a large battery capacity by increasing the energy density of the battery without enlarging a void inside the battery and increasing the area of its negative electrode such that oxygen gas is absorbed at the negative electrode, as in the conventional example, in order to inhibit the pressure in the battery from rising by oxygen gas evolution during the charging.
Still another object is to obtain an alkali storage battery which is superior in charge/discharge cycle performance.